what is meant by resonance frequency ?
I guess you are not asking for formulas or maths here.
If you put a coil and a capacitor either in series with one another, or across each other (parallel), and then check them at different frequencies, at one frequency there will be a change in response.
If the components are in series, and you apply a sinewave to the combination, there will be a very big increase in signal at the junction of the coil and capacitor at one frequency.
This voltage is a sinewave and at the same frequency as the input sinewave but it can be a lot bigger than the incoming voltage in amplitude (especially if you put a small resistor across the signal generator).
If they are in parallel and you apply a signal, at all frequencies except one frequency, the signal will be reduced (especially if you put a resistor in series with the coil and capacitor ) , but at that one frequency the signal will (almost) not be attenuated.
In each case, this one special frequency is called the resonant frequency of the tuned circuit. "Tuned circuit" is the name for a coil and capacitor being used together like this.
This effect is very important because it can be used to select one frequency and reject all others. This let's you listen to one radio station or watch one TV program instead of all of them at once.